I get your point. Although in my opinion not having the level information does actually add something to my playing experience.
What does it add? It keeps me in the game world. When the game starts, I don't know anything about the land or its creatures. In the first couple of minutes I learn about spiders, then wolves. There's a bandit camp nearby. There's a bear cave. This first map exemplifies what I like about this game. Spiders can be beaten easily. Bandits are tricky. Bear is impossible at the start. All on the same map. Which is the only map that can be reached. When I died it wasn't because I was on the "wrong" map. It was because I was a level 1 adventurer who was stupid and a noob and didn't have a clue.
What did I learn from this map? I learned that the world is dangerous. I learned this by dying in-game, not by looking at a number. After the first map I did accept the possibility to encounter something that is actually stronger than my party. This created suspense.
What does this lead to? In my case, I read the monster descriptions in the 'pedia. I also did not dare to engage some scary looking creature, because I was semi-sure I'd get my ass kicked. When I fought that creature later, it was laughably weak.
I also engaged a group of guys that I took for some generic, weak ass bandits. Turned out they were not and I burned through all my spells in one unexpected show of power.
Suprises, suspense, triumph and fear would not have been possible if there was a level number. I'd know beforehand whether or not I am supposed to win this fight. On my 2nd playthrough I will know that spiders are weak to crushing damage and fire. I'll know that this group is dangerous and that group is not. I can focus on efficiency as opposed to exploration.
To put it short: Having numbers takes away exploration and adventure. I like this on my first playthrough.
Your logic is very flawed. The post you are arguing against describes exactly what you claim to want. I'm very confused about your position on this topic. Providing information about the difficulty of a mob is in-game information that your characters would get from evaluating a potential enemy. That is in-game information. Having to fight something, find out it kills you, and then reload is NOT keeping things "in the game world".
Incorrect. You, as a player, may have learned this, but your character certainly did not. You have to reload to a time prior to when your character found this information out (i.e. before the fight). This is NOT keeping things "in the game world" as you claim to like. This is exactly the opposite of that.
Having said that, it would be easy enough to implement the feature as optional, so why not suggest that instead of all the people saying "omg no I don't want this"?